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Seroprevalence of Brucella suis in eastern Latvian wild boars (Sus scrofa)

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BRIEF COMMUNICATION

Seroprevalence of

Brucella suis

in eastern

Latvian wild boars (

Sus scrofa

)

Lelde Grantina‑Ievina

*

, Jelena Avsejenko, Svetlana Cvetkova, Dita Krastina, Madara Streikisa,

Zanete Steingolde, Indra Vevere and Ieva Rodze

Abstract

Brucellosis due to Brucella suis biovar 2 is one of the most important endemic diseases in wild boar (Sus scrofa) popu‑ lations in Europe. The aim of the present study was to determine the seroprevalence of brucellosis in wild boars in the eastern part of Latvia. Wild boars killed by hunters in the period from January to April 2015 (n = 877) and from March to April in 2016 (n = 167) were examined for antibodies against B. suis by the Rose Bengal test (RBT), a complement fixation test (CFT), and by enzyme‑linked immunosorbent assays. In 2015, 199 samples (22.7%) were positive by RBT and/or CFT while 36 samples (21.6%) were seropositive in 2016. Of the Brucella seropositive samples from 2015 and 2016 (n = 235), 162 (68.9%) were also seropositive to Yersinia enterocolitica. Considering cross‑reactivity of serological tests, the seroprevalence of B. suis biovar 2 exposure in wild boars in the eastern part of Latvia was calculated to 14.0% in 2015 and 9.6% in 2016. From selected seropositive samples (42 in 2015 and 36 in 2016) total DNA was extracted and analyzed with an IS711‑based nested polymerase chain reaction (PCR) assay. Species and biovar identification was conducted for bacteria isolated in monoculture from PCR positive samples by species specific primers and Bruce‑lad‑ der multiplex PCR. Brucella suis biovar 2 was isolated from 12/20 samples in 2015 and 9/9 samples in 2016. The aver‑ age seroprevalence was relatively low compared to that found in certain other European countries. Males and females had an equal level of seropositivity, but a positive age‑trend was observed for both males and females.

Keywords: Brucella suis, Brucellosis, Latvia, Sus scrofa, Wild boar

© The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Findings

Porcine brucellosis can be caused by three biovars (1–3) of Brucella suis. Biovar 2 is an important pathogen in wild boars (Sus scrofa) with a broad geographical distri-bution ranging from Scandinavia to the Balkan region [1]. Systematic brucellosis monitoring in wildlife is not demanded by regulatory acts but several studies have reported the presence of this infection in European countries [2–9]. Scientific data on the prevalence of B. suis biovar 2 in the Baltic countries, Russia and Belorus-sia have not been published. A few cases of domestic pig brucellosis have been recorded in Estonia (2006) and Lat-via (2007 and 2008) [10]. The latest outbreak in Latvia was in 2010 in the western part of the country (unpub-lished observations).

Transmission of Brucella bacteria occurs during copu-lation and by consumption of infected birth and abortion products and uterine discharges. Infection is not neces-sarily associated with the presence of gross lesions [11]. Wild boars as well as the European hare (Lepus capensis) are considered as reservoirs for transmissions of B. suis biovar 2 to domestic livestock [1], mainly due to con-sumption of offal from hunted or dead infected hares by wild boars [10].

According to estimates made by the Latvian State Forest Service, the population of wild boars in Latvia increased during the last decades from around 15,000 in 1997 to 74,000 in 2013, but decreased to 49,000 in 2015 due to promoted hunting. The estimated population of European hares in Latvia is 34,700, indicating the poten-tial for transmission of the infection from this host [12].

The aim of the present study was to determine the sero-prevalence of brucellosis in wild boars in the eastern part of Latvia and its correlation to gender and age.

Open Access

*Correspondence: lelde.grantina‑ievina@bior.lv

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Blood and tissue samples (spleen, kidney, tonsil and lymph nodes) were collected from wild boars killed by hunters from January to April 2015 (n = 877) and from March to April 2016 (n = 167) conducted within the national surveillance programs aimed on African and classical swine fever viruses. Hunters determined gender of the animals and age based on tooth eruption pattern (< 12, 12–24, > 24 months). All tested animals were evalu-ated as clinically healthy by hunters and veterinarians, i.e. no obvious clinical or pathological signs of brucellosis were observed.

Samples were transported to the laboratory refriger-ated at 4  °C. Blood samples were transferred to 5  ml tubes, centrifuged and kept at 4  °C until analysis but not longer than 5 days. The tissue samples were kept at

− 20 °C until analysis.

Sera were tested by the Rose Bengal test (RBT) (Rose Bengal assay, IDEXX, Westbrook, USA) and a comple-ment fixation test (CFT) according to the OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals [1] and Standard Operating Procedures of European Union Reference Laboratory for Brucellosis [13, 14]. For the CFT, the following compounds were used: Brucella anti-gens (IDEXX or IDvet, Grabels, France), sheep blood in Alsvers (TCS Biosciences Ltd, Botolph Claydon, UK), rabbit haemolytic serum (TCS Biosciences Ltd), cal-cium–magnesium veronal buffer (IDvet) and guinea pig complement (IDvet). Samples were recorded as seroposi-tive if either the RBT and/or CFT was posiseroposi-tive, and these were further tested with indirect enzyme-linked immu-nosorbent assays (ELISAs) (Ingezim Brucella porcina, Ingenasa, Madrid, Spain) and screened for presence of Yersinia enterocolitica antibodies by an indirect ELISA (Pigtype Yopscreen, Labor Diagnostik Leipzig, Leipzig, Germany).

To identify individuals suitable for Brucella isolation, i.e. animals with an expected high bacterial load, total DNA was extracted from 78 pooled tissue specimens of seropositive animals (strong positive RBT (≥ 1+) and/or CFT (≥ 23.33 IU/ml); n = 42 in 2015 and n = 36 in 2016). Extracted DNA was subjected to IS711-based nested pol-ymerase chain reaction (PCR) [15]. In the case of positive PCR, tissues from 20 animals in 2015 and nine animals in 2016 were subjected to bacteriological culturing. Cultur-ing was done on spleen, kidney, tonsil and lymph nodes separately, according to [1, 16]. Biovar determination was done by further cultivation on selective agar with/ without CO2, the H2S test, growth in the presence of dyes (thionin and basic fuchsin), slide agglutination tests with monospecific A, M, R antisera and lysis by phages according to [1, 17]. These tests were followed by species and biovar confirmation with species specific PCR [18] and Bruce-ladder multiplex PCR [19].

Statistical analyses were done using R program [20] and Chi square test [21]. True prevalence was calculated using EpiTools epidemiological calculators [22].

In 2015, the sampling area covered 40 municipali-ties with a total area of 30,177 km2. A total of 199 ani-mals [22.7%; 95% confidence interval (CI) 20.04–25.58] were seropositive for Brucella. In 2016, the sampling area covered 18 regional municipalities with a total area of 18,461 km2. Of the 167 tested animals, 36 (21.6%; CI 16.0–28.4) were serologically positive for Brucella. Data of both study years are combined in Table 1. In 2015, 130 (65.3%) of the seropositive animals were also sero-positive to Y. enterocolitica, while this was the case for 32 (88.9%) animals in 2016 (Table 2). Due to cross-reactivity between B. suis and Y. enterocolitica in serological tests, serology data were combined with PCR results revealing a prevalence of B. suis biovar 2 infection of 14.0 and 9.6% in 2015 and 2016, respectively. This level of exposure to B. suis is relatively low compared to that in certain other European countries. Serological surveys have reported the proportion of seropositive animals estimated by microagglutination test and CFT to be as high as 15.0% in the Czech Republic (1995–1996) [3], while estimations using RBT and CFT were 19.7% in Italy (2001–2007) [6], and 22.6–29.4% in Croatia (1996–2000), and estimations based on ELISA were 22.0% in north-eastern Germany (1995–1996) [5], up to 39.6% in some cantons of Swit-zerland (2001–2003) [8], and on average 24.4% in Poland (2012) [9]. Among these investigations the cross-reactiv-ity problem of serological tests with Y. enterocolitica was assessed only in Germany [5].

The density of seropositive wild boars in 2015 ranged from 0 to 5 animals per 100  km2 (Fig. 1). The num-ber of tested animals ranged from 0.2 to 12 animals per 100 km2. The regions with the highest numbers of sero-logically positive animals (2–5 per 100 km2) where those with relatively high numbers of hunted and tested ani-mals. The seroprevalence in these regions ranged from 25.0% (in Baltinavas and Rujienas regions) to 42.4% (Nauksenu region). The density of seropositive wild boars in 2016 ranged from 0 to 0.5 animals per 100 km2. The number of tested animals ranged from 0.1 to 3.8 animals per 100 km2 (data not shown).

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Table 1 S er ologic al pr ev alenc e f or Bruc ella suis in

the wild boar popula

tion in the east ern par t of L at via in 2015–2016 G ender A ge ca tegor y Total < 12 mon ths 12–24 mon ths > 24 mon ths Unk no wn

Number of animals/ positiv

e

Pr

ev

alenc

e (%)

Number of animals/ positiv

e

Pr

ev

alenc

e (%)

Number of animals/ positiv

e

Pr

ev

alenc

e (%)

Number of animals/ positiv

e

Pr

ev

alenc

e (%)

Number of animals/ positiv

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Table 2 Comparison of nested polymerase chain reaction results with the Rose Bengal test, complement fixation test, and indirect enzyme-linked immunosorbent assays of swine brucellosis and Y. enterocolitica in 2015 (n = 42) and 2016 (n = 36)

RBT, Rose Bengal test; CFT, complement fixation test; ELISA, Ingezim Brucella porcina; ELISA Y. enterocolitica, Pigtype Yopscreen; Pos, positive; Neg, negative; NT, not tested

PCR result RBT 2015/2016 CFT 2015/2016 ELISA 2015/2016 ELISA Y.

enterocol-itica 2015/2016

Pos Neg Pos Neg Pos Neg Pos Neg

Positive 24/18 NT 19/16 5/2 23/18 1/0 10/16 14/2 Negative 18/18 NT 18/9 NT/7 18/14 NT/4 16/15 2/2

Fig. 1 Average density of infected wild boars estimated as serologically positive animals per 100 km2 (2015). Regions: 1—Erglu, 2—Ilukstes, 3—

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The prevalence of seropositive boars did not differ between genders. The prevalence of seropositive boars was positively correlated with increased age irrespec-tively of gender for 2015 samples (χ2= 14.6, P = 0.0007 for males and χ2= 6.26, P = 0.04 for females). Positively correlated seroprevalence among age groups have been observed also in other countries, for example, in Italy [6], and a sex/age interaction was found in Spain [23]. In 2016 differences between age categories were statistically not significant probably due to the low number of tested animals in this year.

Statistically significant differences were obtained also by sampling month in 2015 (χ2= 17.6, P = 0.0005) with the highest prevalence in April (36.8%). Significant differ-ences among sampling seasons have been recorded also in other investigations, for example, in Spain in relation to hunting activities [23].

The seroprevalence in some regions of eastern Latvia was 25.0–42.4%. These areas corresponded to regions with the highest percentage of forest area (57%) com-pared to the average forest area of 50% in Latvia in general [12]. A high degree of forest area is probably positively correlated with a high density of wild boars. It has been estimated that 89% of the territory of Latvia contains habitat suitable for wild boars [24]. The average seroprevalence for B. suis in Latvian wild boars seems to be relatively low in comparison to certain other European countries but still the wild boar population has to be con-sidered as an important reservoir for the B. suis biovar 2 transmission to domestic pigs.

Abbreviations

CI: confidence interval; CFT: complement fixation test; ELISA: enzyme‑linked immunosorbent assay; LPS: lipopolysaccharide; NT: not tested; PCR: polymer‑ ase chain reaction; RBT: Rose Bengal test.

Authors’ contributions

IR and DK planned the study. ZS and IV carried out the serological tests. JA and MS did the microbiological analyses. SC performed the molecular tests. LG‑I prepared the manuscript. All authors read and approved the final manuscript.

Acknowledgements Not applicable.

Competing interests

The authors declare that they have no competing interests.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Consent for publications Not applicable.

Ethics approval and consent to participate Not applicable.

Funding

The study was financed by the Ministry of Agriculture of Latvia within the state budget program [Grant Number 20.02.00]. The sponsors were not involved in study design, in the collection, analysis and interpretation of data, in the writ‑ ing of the report, and in the decision to submit the article for publication.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in pub‑ lished maps and institutional affiliations.

Received: 19 September 2017 Accepted: 15 March 2018

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Figure

Table 2 Comparison of nested polymerase chain reaction results with the Rose Bengal test, complement fixation test, and indirect enzyme-linked immunosorbent assays of swine brucellosis and Y

References

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